[SI-LIST] Re: Insertion Loss, Return Loss, ILD, UNi versus Dual stripline

  • From: "Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>
  • To: "Haller, Robert" <rhaller@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 5 Aug 2010 10:27:29 -0500

Robert,

I'll toss in a few thoughts on the topic - eager to hear counter
arguments.

The premise of the same impedance in the two cases suggests that the S11
(return loss) must be the same as well.  That said, you will likely
experience some differences when measured in the lab due to launch
effects, but these should be relatively minor in a well controlled set
of structures.  Theoretically, though, the same impedance (that's
complex impedance with real & imaginary parts) will produce the same
S11. A quick review of how S11 is defined will demonstrate that.

For S21 (insertion loss), the premise that the metal geomtries and
dielectric loss tangent are the same suggests that S21 should be similar
as well.  However, there will be some minor difference between them due
to return path effects and current crowding (i.e., skin effect).

In brief, the single/balanced stripline case will have a mostly-uniform
distribution of current around the surface of the stripline, because the
ground return paths are at equal distances from the top and bottom
surfaces of the stripline.  Also, the return current in the referenced
ground planes will be equal between the two planes.

In the dual/offset stripline case, the top reference plane is closer to
the stripline than the bottom reference plane (or vise versa for the
bottom signal).  Because of this asymmetry, the current in the stripline
will crowd near the top of the stripline.  Similarly, the return current
in the top ground plane will be greater than in the bottom ground plane.
This crowding and imbalance will affect the AC (frequency-dependent)
resistance, which will degrade S21 more so than in the single/balanced
stripline case.

Taking the case to the limit, you could stretch out the dual/offset
stripline case to the point where it is the same as an embedded
microstrip.  The same philosophy holds and can readily be modeled and
measured.

That said, the difference between the two cases should be relatively
small.

$0.02,
Pat Zabinski
Mayo Clinic


 
> ASCI Graphics did not come through correctly- will try again. 
> I know the first option is preferable, and I know I can (and 
> have) measured the Loss. What I am trying to quantify is 
> Specifically Which S parameter(s) I can use/measure to 
> quantify the difference;
> Insertion loss, Return Loss, or insertion loss deviation 
> 
> GND -----------------
> Sig        -  -   
> GND -----------------
> 
> GND -----------------
> SIG     - - 
> SIG           - - 
> GND -----------------
> 

.
.
.

> 
> I have 2 Diff pairs in the same dielectric Material 
> (ISO370HR). One is Uni Stripline, One is Dual Stripline. 
> Aside from the potentially different core/Prepreg material, 
> resin content, glass weave (etc), and they are the Same 
> Copper Geometry (say 4 mil line/7 mil space) and have the 
> same impedance, same DC resistance  will I be able to observe 
> a difference in Insertion Loss, Return Loss, or Insertion 
> Loss Deviation ?
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